This disclosure relates to the field of pressure control of wells drilled through subsurface earthen formation. More specifically, the present disclosure relates to maintaining wellbore pressure in the event of certain drilling conditions, such as drilling fluid lost to an exposed subsurface formation and the influx of gas into the well from a formation.
U.S. Pat. No. 7,562,723 shows one example embodiment of a “managed pressure drilling” control system. The system shown in the '723 patent may be used to maintain a selected pressure in a wellbore while drilling fluid pumps (rig mud pumps) are operating and while such pumps are switched off for purposes such as adding or removing a segment (“joint” of “stand”) of a drill string. The system shown in the '723 patent comprises logic operable to detect influx of fluid into the well from a subsurface formation as well as loss of fluid from the well into a subsurface formation. The system shown in the '723 patent may be used with land-based drilling as well as marine drilling (i.e., drilling subsurface formations below the bottom of a body of water).
U.S. Pat. No. 8,413,724 issued to Carbaugh et al. describes a “riser gas handler.” In the event of influx of gas into a well during marine drilling, where a “riser” connects a subsea well control apparatus to a drilling platform on the water surface, the gas expands in volume as it travels upwardly through the liquid column in the riser. As the gas expands in volume, the hydrostatic pressure exerted by the fluid column in the riser is reduced, and the pressure in the well may correspondingly increase. The pressure increase in the riser may at some point exceed the pressure bearing capacity of the riser. The device shown in the '724 patent is intended to divert fluid in the riser that contains gas to flow lines external to the riser. Such flow lines may have a pressure capacity much greater than that of the riser, thus enabling the gas to be removed from the well using known procedures to stop influx of fluid into a well from a subsurface formation.
Because subsurface formation fluid pressures can change substantially and unpredictably, it is desirable to automate systems such as those described above in the Reitsma and Carbaugh et al. patents. More specifically, such automation may be applicable to and coordinated with both such systems as well as with a well pressure control apparatus (blowout preventer—“BOP”) disposed proximate the water bottom and connected to the base of the riser.